This disclosure relates to a multi-stage pump, and particularly to a multi-stage gear pump assembly used as a fuel pump in an aircraft gas turbine engine. It will be appreciated however, that the disclosure may find application in related environments and applications that encounter the same issues.
A large portion of aircraft engine operation involves cruise and idle situations which do not demand large quantities of fuel flow. However, certain circumstances require additional flow, for example during takeoff, climb, or windmill re-light. The fuel pump assembly must be able to satisfy both demands, while adequately addressing associated parameters such as fuel pump size, efficiency, etc. For example, it is known to employ multiple stages of a positive displacement pump assembly to meet the different needs of the aircraft engine to improve efficiency over traditional single stage gear pumps. Typically first and second stages of a multistage gear pump are selectively used. Thus, second gear pump stage is designed to handle the cruise and idle operations of the aircraft while the first gear pump stage is selectively employed in conjunction with the second stage pump to meet the higher demand modes of engine operation.
Inclusion of independent gear pumps in the same housing raises a number of issues. For example, when the second pump is functioning at maximum capacity, the first gear pump is operated at a reduced pressure state to reduce energy consumption. In the reduced pressure state, the first pump has a tendency to become unstable. As a result of the teeth of the gears transferring the relatively low load, there is resultant tooth bounce and instability, which could ultimately lead to gear tooth failure. Ideally, a full fluid film without any physical contact between the journal and the bearing surfaces is desired in the bearing assembly. This gear instability can prematurely wear the journal bearing. The bearings that support the arrangement can also become unstable when minimizing pressure to the first pump. A phenomenon known in the industry as bearing oil whirl can occur in journal bearings that are lightly loaded, which could ultimately lead to bearing failure. A conventional arrangement, for example, may drive the second gear stage through the tooth mesh of the first gear stage in order to alleviate the above issues with tooth instability and bearing oil whirl. This results in increased loading on the gear teeth of the first pump, which would require an increase in the gear teeth size or count and increasing weight for example.
There are also issues with selectively switching between single and multistage use of the pump. For instance, different forces and stresses result from different modes of operation of the multistage pump. Changing or turning the pressure on and off in connection with one of the gear pump stages has a resultant impact on the stability and efficiency of the pump assembly gears and bearings.
There is always a need to reduce the weight and overall envelope size of the pump assembly. Thus, a conventional arrangement where the first and second gear pumps are offset from one another may address a portion of the issues associated with one pump being independent of the other, but it unnecessarily adds additional components, additional wear, additional weight, and increases the overall size of the multistage pump assembly.
Accordingly, a need exists for an improved multi-stage pump assembly that addresses these needs and others in a reliable, economic manner.